Various systems have heretofore been proposed for using telephone lines to transmit alarm signals from a transmitter unit in a home, office, school, factory or the like to a receiver unit in a fire, police or security office. In a number of such systems, the transmitter unit includes dialing apparatus operated in response to an alarm condition for dialing a telephone number assigned to a receiver unit and establishing a connection thereto through the telephone lines and telephone exchange circuits. In addition, such systems have included apparatus for transmitting data through the connection thus established. Some of such proposed systems would not provide proper security against the possibility of interference with their proper operation by unauthorized persons. For example, systems have been proposed in which the data is transmitted in a format such that it would be quite easy for an unauthorized person to "jam" the system by providing apparatus for transmitting signals to the receiver unit to tie it up and make it impossible to determine whether an alarm condition existed at the location of any particular transmitter unit.
Other types of systems have been so complex as to have questionable reliability as well as being difficult and expensive to manufacture. For example, systems have been proposed using multiple transmissions back and forth between transmitter and receiver units and other systems have been proposed using random signal generators.
The construction and operation of the prior-proposed systems has otherwise left much to be desired. Arrangements for dialing or for storing or determining information to be transmitted have been cumbersome and have included, for example, switches operated by motor-driven cams, magnetic tape arrangements, complex arrangements of multivibrators, counters and gates or pulse transformers in a matrix arrangement, multiplex switches operated from a clock circuit and memories controlled from a plurality of counter circuits.
Also, many of these systems have been deficient in that they can be rendered ineffective if no dial tone is received from the telephone exchange circuits or if a busy signal is sent back, or if there is outside interference at the time of transmission of data.